Non-contact communication devices are widely used in a range of applications, such as stock control and inventory management, item tracking, security, and the like. Since a basic function of non-contact communication devices is identification of the device or tag, and radio frequencies are used, this technology is also known as RFID (radio frequency identification devices), although should be noted that applications now considerably extend beyond mere simple identification function. Hereinbelow, non-contact communication and RFID shall be used synonymously.
RFID generally has two main components: a device, which may be, for instance, in the form of a tag, or embedded in a card, chip, or other item, and a reader. Hereinbelow, the term tag will be used to indicate any configuration of the device. In a typical arrangement, the reader transmits a RF AC signal at a frequency which may typically be in the range of 125-148.5 kHz (low frequency—LF), around 13.56 MHz (high frequency—HF), or between 300 MHz and 3 GHz (ultrahigh frequency—UHF). The exact frequency ranges used depend on the regulatory requirements of the country or region for which the reader and tags are designed. The tag typically modulates the RF signal, and retransmits or backscatters it back to the reader. The reader detects the return modulated signal, and demodulates it in order to extract information from the tag. In more advanced non-contact communication devices, the reader may provide information to the tag within the RF signal, and the tag may process or store this information and may provide a response thereto.
Tags may be either passive, or active. An active tag has its own power supply, to power for instance the modulation circuitry. However, providing an on-board power supply is relatively expensive and they are relatively bulky: therefore, passive tags are more widely used. Passive tags do not have their own power supply, but rather extract power from an external source, which is most typically the RF field. Although this invention relates primarily to passive tags, it may also find application in active tags, for instance to avoid or reduce Bit Error Rate (BER) losses under detuned conditions.
Passive tags, and in particular passive UHF tags, generally have a higher “read range”—that is to say, the distance between tag and reader over which the tag can communicate—than other tags. However, for all tags, and passive tags in particular, the read range can be affected by environmental factors, which may detune the tag, thus modifying its operating frequency and potentially reducing the received power.
An RFID tag's performance, assuming constant power consumption in its integrated circuit (IC), depends on the amount of power that can be captured by the antenna and provided to the IC. This in turn depends on impedance matching between the antenna and the IC. Environmental factors such as the presence of metals, liquids or other materials, or the close proximity of further tags, may lead to absorption or parasitic capacitance, which can the result in detuning the tag. Detuning of the tag may also result from the process spread in manufacturing of, for instance, any of the chip, antenna or packaging.
It has been proposed in U.S. Pat. No. 7,167,090 to provide a feedback tuning circuit, to mitigate the problem of detuning. The tuning circuit alters the impedance of the impedance matching network coupling the antenna to rest of the device, in order to maximise the RF input signal and thereby optimise the performance of a power extraction circuit.
However, such a circuit is sensitive to noise on the amplitude modulation (AM) signal, variation in transmitter power, and changes in the surrounding environment of the tag.